Narrow Results

The properties of sI methane hydrate dissociation at different temperatures are investigated using molecular dynamics (MD) simulations, focusing on the characteristics of structure of melting water that has memory effect. Upon melting, the clathrate structures of hydrate are damaged. The density of dissolved methane decreases as the melting temperature rises. There is a positive correlation between the density of dissociated water molecules and melting temperature. Most oxygen atoms of dissociated water molecules remain tetrahedrally coordinated whereas the hydrate-like torsion angles (H–O–O–H) are like that of normal water. Therefore, the tetrahedrally coordinated oxygen atoms are one of the factors contributing to the memory effect.

The interfacial properties of hydrate and its ambient play an important role in hydrate technique. In this paper, the molecular characteristics of H₂O in hydrate/ice/liquid water mixture system are investigated based on molecular dynamics (MD) simulations. The structure I (sI) methane hydrate is partially heated to obtain the studied system. The properties including hydrogen bond, radial distribution function (RDF) and F3 order parameter (tetrahedral coordinated parameter of H₂O) indicate that there is little difference of water structure in the hydrate region and ice/liquid water mixture region. The F4 order parameter (parameter based on H–O–O–H torsion angles of H₂O) could be used to distinguish the different region. The value of F4 experiences the continuous change at interface between mixture region and hydrate region.

Gas hydrate is a nonstoichiometric crystalline material. It is attracting more and more attention not only because natural gas hydrate is a potential energy resource, but also because hydrate-based technologies can be potentially applied to the industry of energy and environment such as energy storage, carbon sequestration, etc. The nucleation mechanism of gas hydrate is a key issue in studying reservoir formation of natural gas hydrate and the kinetic properties of gas hydrate as an energy-storage media. Molecular dynamics simulation can provide microscopic insights into molecular-scale understanding of hydrate nucleation. In this review, the advances on the study of heterogeneous hydrate nucleation by molecular dynamics simulation are summarized, with a specific focus on the factors affecting nucleation processes. The interfacial energy barrier for nucleation will be lowered by molecular structure of interfacial water between hydrate and solid substrate. The interface and surface properties of solid substrate, including hydrophilicity and hydrophobicity, surface roughness, layer-accumulated adsorption potential, crystallinity and surface layer-charge are also recognized as being associated with the process of hydrate nucleation by influencing the dynamics of gas and water molecules. In case of hydrate nucleation in confined pore space, confinement effect, pore size and elasticity are identified as the controlling factors.

Natural gas consists mainly of methane (CH₄) and ethane (CH₃CH₃) and other hydrocarbons in small quantities. Depending on the source it may also contain CO₂, nitrogen (N₂), hydrogen sulfide (H₂S) and helium (He). It is very abundant and its combustion contributes to the increase of the CO₂ content in the biosphere. Natural gas is a non-renewable fuel (Section 5.1). It is widely used to produce electricity, heat, chemicals including hydrogen, and hydrocarbons (liquid fuels) and as a fuel for motor vehicles…